1. Field of the Invention
This invention relates to a liner setting assembly for use in setting a liner into an oil well.
2. Background of the Invention
A casing string is made up of sections of pipe threadably fastened together and is used to line an oil well. The casing string is designed before drilling commences, but this is an exacting task and, because of the difficulty of designing, ex situ, a casing precisely in accordance with the requirements of any particular well, most deep wells include one or more liners.
A liner is a casing string which is run into the production area of a well to protect the face of the formation and prevent sand or debris from flowing into the well. A liner is usually hung using a liner hanger set in the lower section of the last casing string in the well.
The predominant reasons for using liners are (1) that the well is to be drilled deeper than originally forecast, or (2) that ground formation pressures higher than originally anticipated are encountered.
A tie back receptacle is used to extend a liner to the well head. It enables a damaged or worn casing above a liner to be repaired and provides an added measure of protection against corrosion or pressure.
An existing liner assembly utilizes a "floating cage" which is provided with four drag springs and which acts as a liner hanger. The cage is provided with four arms each of which has a slip for securing the hanger to the previous casing string. Each slip has serrated edge or grip on the side facing the internal wall of the previous casing string. The other side of the slip is smooth so as to enable the liner to slide relative thereto. The liner has welded to its distal end a frustoconical portion. The liner is inserted into the well using a setting tool and setting string (known in the art) until the hanger starts to take its weight (with the frustoconical portion resting on the slips and urging these into contact with the casing string). The liner is then inserted further until a small proportion (e.g. about 1,500kg) of its weight is carried by the hanger and cement is pumped into the annular gap between the liner and the well via a setting shoe at the lower end of the liner to secure the liner in place. During insertion of the cement, the liner is reciprocated and rotated to aid bonding. At this stage, the hanger is not yet set. The term `set` used herein indicates that the full weight of the liner has been applied to the hanger. When cementing has been completed, reciprocation and rotation is stopped and the full weight of the liner is allowed to rest on the hanger. Hopefully the hanger is now set. The setting string is now used to apply a setting weight (e.g. 2,500kg) to the liner. The setting tool is then removed.
A problem arises in that there is no guarantee that the hanger is actually correctly set in position. Further, it is not possible to move the liner once the hanger has been set since the need to reciprocate and rotate the liner means that the cement must be introduced around the liner prior to setting the hanger.